1. Field of the Invention
This invention relates to an improved process for the preparation of diphenylamine or a nucleus-substituted derivative thereof (hereinafter referred to as "diphenylamine and a derivative thereof" for the sake of brevity). More specifically, this invention relates to a process for the preparation of diphenylamine or a derivative thereof by using phenol or a nucleus-substituted derivative thereof (hereinafter referred to as "phenol or a derivative thereof" for the sake of brevity) as a hydrogen acceptor and reacting, in the presence of a hydrogen transfer catalyst, aniline or a nucleus-substituted derivative thereof (hereinafter referred to as "aniline or a derivative thereof") with cyclohexanone or a nucleus-substituted derivative thereof (hereinafter referred to as "cyclohexanone or a derivative thereof") corresponding to the phenol or the derivative thereof through condensation and inter-molecular hydrogen transfer. In particular, this invention is concerned with an industrially advantageous process for the preparation of diphenylamine or a derivative thereof by using a nucleus-substituted derivative of aniline as a raw material.
2. Description of the Related Art
Diphenylamine and derivatives thereof are useful compounds as intermediates for the production of dyes, agricultural chemicals, medicines, compounding agents for rubber and so on. For instance, 2-methyl-4-alkoxydiphenylamine, which can be obtained from a nucleus-substituted derivative of aniline, is highly valued for its usefulness as a raw material for fluoran dyes employed in pressure-sensitive or heat-sensitive recording papers.
Diphenylamine or a derivative thereof has heretofore been prepared by deammoniation of aniline or a derivative thereof; dehydration of a reaction product between aniline or a derivative thereof and phenol or a derivative thereof; or dehydrobromination of an reaction product between aniline or a derivative thereof and dibromobenzene.
Preparation of diphenylamine or a derivative thereof has also been proposed in patent publications, for example, in Japanese Patent Laid-Open Application No. 49924/1974 in which N-cyclohexylideneaniline is reacted in a vapor phase with an oxygen-containing gas in the presence of an oxidizing catalyst such as silica; in Japanese Patent Publication No. 14738/1974 in which phenol and aniline are reacted using .gamma.-alumina as a catalyst; and in Japanese Patent Application Laid-Open No. 58648/1982 in which, upon preparing diphenylamine or a derivative thereof while reacting, in the presence of a hydroreduction catalyst, an amine and cyclohexanone or a derivative thereof to form a Schiff base, that is, an intermediate such as N-cyclohexylideneaniline, the synthesis is conducted using styrene or a derivative thereof as a hydrogen receptor.
These conventional processes are, however, accompanied by one or more drawbacks such as the need for complex reaction steps and/or low reaction velocity.
It is also disclosed in Japanese Patent Application Laid-Open No. 193949/1985 that diphenylamine or its derivative is prepared by reacting cyclohexanone or a derivative thereof with aniline or a derivative thereof in the presence of a hydrogen transfer catalyst while using phenol or a derivative thereof as a hydrogen acceptor and forming the cyclohexanone or the derivative thereof in a reaction system. This patent publication also discloses a process for obtaining 2-methyldiphenylamine in a yield of 97.0% (selectivity: 99.3%, conversion: 97.7%) by using 2-methylaniline and an excess amount of phenol in the presence of a palladium catalyst. This process is generally satisfactory from the viewpoints of the reaction velocity and selectivity. Industrial adoption of this process, however, is accompanied with the problems that phenol or a derivative thereof has to be used in large excess relative to aniline or a derivative thereof to suppress by production of N-cyclohexylaniline and the like and the expensive noble metal catalyst has to be added freshly in a large amount upon recovery and recycle of the catalyst.